General Motors’s quest to sell its Hummer division may get a bit tougher after Tuesday.That is when Dan Towbin–whose dealerships grabbed national attention as the backdrop for an A&E television series called “King of Cars“–will close his Hummer dealership, one of the brand’s largest stores in the U.S. and the only one in Las Vegas. It is at least the eighth Hummer dealer closing this year, nearly a 5% decline in the brand’s U.S. dealer base. As GM tries to sell the Hummer brand, dealership closings are one of a handful of black marks the auto maker carries into negotiations with potential buyers. Critics have suggested it will be hard for GM to sell Hummer, and there is little good news for the brand–at least in the core U.S. market–to counter that skepticism.